Why does added weight to the back of my eBike make it more stable?

[momus3]

This is how the bike looked, and it was more stable this way compared to w/ no stuff in the back. The groceries must have weighed 10 or 20 lbs.

Could it have been the two bags on the side, which hand low? They were very light though compared to the weight in the basket. The steering, although lighter, was much more stable. Any movement of the handlebars and it acted smoothly and corrected itself. It reminded me of being on a motorcycle rather than riding a top heavy eBike, which is how it normally feels unloaded w/ groceries. Could have rode the bike w/ no hands easily. Seeing this, I'm thinking of putting one of those flat batteries in the rear rack.

Any ideas why adding a significant amount of weight well above the bike's center of gravity and far to the rear (where it's already heavy due to the hub motor) makes it more stable and feel much safer to ride? It was also a much smoother ride, probably due to the rear tire being squished more. I made some panic stops and fast quick turns as an experiment, and it definitely handled better than w/o the added weight. I'm gonna load it up later w/ a bunch of canned pinto means and see how it goes w/ and w/o the outrigger bags.

 

[Tommm]

The more weight up top, the more difficult it is to move the bike side to side.

 

[momus3]

I never thought of that. Thanks.

Here's my test results: with 10 lbs in the bottom of the rear rack = no significant increase in stability.

with 10 lbs in the rear rack and 6 lbs in bags on either side of the rear rack hanging like panniers = more stability.

with 10 lbs in the rear rack and the two 6 bags (22 lb total) on top of them = only minimal increased stability

Looks like the answer is to put a flat battery in the bottom of the rear rack, or under it, to more than make up for the loss of range due to having a heavier bike, and put panniers or narrow wire baskets on either side and experiment w/ small amounts of ballast to get the optimal handling. A rear tire that's rated for more weight is in order too. I honestly don't 100% understand all the physics behind all this, but it doesn't matter, it is what it is. Bike rides much nicer w/ the extra weight.

 

[MadRhino]

Steep steer angle makes poor stability. Adding weight on the rear does sort of a compensation by lightening the front, but it is a bad solution to a geometry problem. Adding weight to a bike is never the proper way to balance it. Moving the center of gravity by angle mods and/or cockpit placement is the adequate tuning.

 

[markz]

I always hated putting my battery on the rear rack with a rear hub motor, it was too top heavy. Battery weight plus hub motor weight, plus me, plus whatever Im carrying which is usually on the handlebars.

 

[dogman dan]

I think Mad Rhino hit it spot on. But at some point, which I believe to be about 10 pounds, you leave stable behind and start to get a lot of frame flex side to side, causing high speed wobble at lower and lower speeds as loads approach 50 pounds.

Looks like you found though, that your particular bike is less stable without weight in the rear. This could be an advantage for off road riding, which your frame is designed for. Kind of like fighter planes are made inherently unstable, so they can flick into erratic turns quicker.

For street ride stability, get a street bike, ideally a cruiser. Amazing what a slacker head angle, and two inches longer wheelbase does for how it feels in a turn.